Activated or inactivated? Transfusing the right platelets

Performed in about five minutes, the test uses laser light to illuminate a sample and identify the shape of platelets as well as the microparticles, “maybe even small aggregates,” she said. As the light bounces, or scatters, off the platelets and particles, it provides information on the speed of the particles based on their movement in suspension.

“It’s very similar to the radar technology the police use to detect speeding cars, and in our situation the speeders are the microparticles.” The microparticles are very small, “and therefore they are fast compared to the platelets or maybe the even larger activated and microaggregate platelets combined.”

The device plots the size distribution of the particles using the scattered light information. If the microparticle concentration in the sample is less than 15 percent, it is classified as nonactivated, or resting. If it is composed of more than 15 percent microparticles, it is considered activated.

There are many ways to test for platelet activation, one of which is to use the microscope, which Dr. Maurer called “subjective and cumbersome.” CD62 expression on the surface is a research test. Blood cell counters cannot detect the microparticle sizes that the ThromboLux can, she said.

Flow cytometers can identify the smaller particles, Dr. Maurer noted, but a number of factors should be considered, including “gating and setting thresholds,” which is currently not standardized. And not every flow cytometer can be used to identify microparticles, she said. “The International Society for Extracellular Vesicles does say one needs very specific flow cytometers to identify particles as small as microparticles, which have an average diameter of about 200 nm.

“But we have done comparative studies with ThromboLux and flow cytometers, and we did see high correlations, even though the very small microparticles might not be detected with a flow cytometer,” she said (Xu Y, et al. Transfusion. 2011;51[2]:363–370; Labrie A, et al. Transfus Med Hemother. 2013;40[2]:93–100). “But in terms of seeing whether platelets are activated or not based on the fragmentation, it still works on flow cytometers.”

If platelets are stored at room temperature with agitation for five to seven days, why does 36 percent of the inventory, or in the University of Colorado’s case, 33 percent, become activated? “It really comes from the donor,” Dr. Maurer said. Platelets are immune cells and hemostatic cells, “so there are a lot of things they have to do in the donor’s circulation.”

“When the donor has anything—from allergies, asthma, depression, stress—the donor might donate activated platelets, and the separation process could have an additional effect on whether or not those platelets are stressed and more activated.” In a donor whose platelets are activated, the platelets will be more vulnerable to additional stress, more fragile, and will be even more activated once in the bag. She compares it to putting glasses into a dishwasher: “Intact glasses won’t mind that stress, but a glass with a crack might actually break.”

A very healthy donor will have resting platelets that are resilient to additional stress and will remain resting after processing.

Dr. Maurer shared an example from one of the other study sites, the University of Kansas Medical Center. In one patient with AML who received seven platelet transfusions in that study, a single activated platelet transfusion with an approximately 25 percent microparticle concentration reduced 18-hour platelet count increments from an average of about 80 × 10⁹/L following the first three nonactivated platelet transfusions to a mean of about 35 × 10⁹/L over the subsequent four transfusions, after administration of only a single activated platelet product.

The higher response to the resting platelets than to the activated platelets “is really interesting,” Dr. Maurer said, “because we are used to thinking that if we give resting platelets, we should immediately see some positive outcome.” She asks: “Is there an explanation based on the immunology of the patient and the response of the patient to transfusion?”

Atypical hemolytic uremic syndrome is an example of a situation in which a patient might have an immune response to platelets, Dr. Maurer said, pointing to a publication that outlined this process (Peerschke EI, et al. Adv Exp Med Biol. 2008;632:81–91).

“A triggering event can activate the platelets, and the immediate immune response would be that the platelets get opsonized by complement and then recognized by the macrophages and therefore removed,” she explained. “Interestingly, the macrophages then become antigen-presenting cells, and they can trigger T-cells and B-cells subsequently to make antibodies, which, with a subsequent trigger event, would cause the platelets to be very quickly removed.”

In a situation in which activated platelets come from the donor and are transfused to a thrombocytopenic patient, they become opsonized and removed from circulation, Dr. Maurer said. “But they could also trigger this response to produce antibodies, and when antibodies already exist, the platelets get removed quickly, even if not activated. So in a situation of HLA or HPA mismatch, that could still be the case.”

The meta-analysis of the four platelet activation studies conducted at the University of Kansas Medical Center, UC Health Denver, Children’s Medical Center Dallas, and Vancouver General Hospital reveals that when patients receive activated platelets, their count increments are lower by an average of about 5,000 platelets/μL. While this may not seem like much, Dr. Maurer said, the fact that the time between transfusions also decreased by an average of 13 hours is an indication that the clinicians thought it to be important.

“We know that most clinical decisions are made based on the platelet count for patients who receive platelets prophylactically,” she said.

Results from the four sites demonstrate that “despite the differences in platelets, despite the differences in patient population, any kind of differences in SOPs, we see a decrease in the clinical outcome both in count increment as well as time between transfusion when hematology/oncology patients receive activated platelets.” (Maurer E, Noland DK, Kniep J, et al. Platelet activation affects transfusion outcomes in hematology-oncology patients: meta-analysis of data from four North American hospitals. Poster to be presented at 2019 AABB Annual Meeting; October 2019; San Antonio.)

Not surprising, she said, in that the regulatory measures put in place decades ago to minimize activation are there so the best outcome can be obtained for cancer patients. Now, knowing that a high percentage of activated platelets are in inventory, “we have to screen so that the resting platelets, as was intended, go to cancer patients while the activated platelets go to the actively bleeding patients in surgery and trauma.”

With transfusion frequency increasing after activated platelets are administered, it’s not only a matter of cost, Dr. Maurer said, but also workload. “It increases how many platelets have to be coming into inventory, have to be released, have to be tested.” And though cost-related studies are still being done, “the prognosis is that platelets are probably 10 to 20 times more expensive in terms of their administration cost compared to the purchasing cost.”

If the time between transfusions declines because they’re less efficacious than they could be, Dr. Maurer said, “there’s more work.”

Hence the benefits from what she calls the personalized platelet transfusion. “It has been in the literature and the knowledge of transfusion medicine for a long time that resting platelets should go to cancer patients and the others to trauma and surgery patients”—to reduce usage, cost, and workload and to give the patient what is best.

David Wild is a writer in Toronto.